Skin packaging is essentially a vaccuum forming process. In a typical process, a sheet of thermoplastic film is placed in a frame, and below the frame is a vacuum plate upon which a piece of support pervious to gases such as a backing board is placed. The product to be skin packaged is positioned on top of the backing board and heat is applied to the thermoplastic film in the frame. When the film has been heated to become sufficiently soft, the frame is lowered and the plastic sheet drapes itself over the product. As this happens, a partial vacuum is created through the vacuum plate and the air underneath the plastic film is withdrawn through the backing board. The air pressure differential between the top and the bottom of the plastic sheet causes the sheet to be tightly pressed around the product. The support is adapted to bond to the plastic sheet. Typically, the film may be thermally bonded to the support, or the film may be coated with a glue or the backing board may be so coated. Where the two contact each other, a strong bond is formed resulting in a package in which the product is tightly held to the support for safe shipping and for subsequent rack display in retail stores.
Vacuum skin packaging differs from the above described skin packaging process in that both the thermoplastic film and the support are impervious to gases and the resulting package can be evacuated and hermetically sealed, if desired. The same end result is sought, i.e. the product is to be tightly held by the film to the backing board. The vacuum skin packaging process generally employes a vacuum chamber with an open top. The product on an impervious backing board is placed on a platform within the vacuum chamber. The top of the chamber is then covered by a sheet of film which is clamped tightly against the chamber to form a vacuum type closure. The chamber is evacuated while the film is heated to forming and softening temperatures. The platform can then be raised to drive the product into the softened film and air pressure can be used above the film to force it tightly around the product.
The conventional method employes a backing board which is porous or which is perforated to make it pervious to gases so that the vacuum may be drawn directly through the backing board.
The term "vacuum" as used herein means a differential fluid pressure where the fluid can be either a gas or a liquid. The term "skin packaging" as used herein is intended to refer to vacuum packaging processes as described above, regardless of whether the support is pervious or impervious. Typically the support is made of a cardboard coated with adhesive polymer so that the film sheet will adhere as the sheet is heated and draped over the article being skin packaged and a differential pressure is applied.
Various patents describe skin packaging machinery and methods. Representative patents are French Pat. No. 1,258,357 (Bresson) (Mar. 6, 1961), French Pat. No. 1,286,018 (Laroch Freres, Ltd.) (Jan. 22, 1962), Australian Pat. No. 245,774 (Colbro Proprietary Ltd., and Cole and Son Proprietary Ltd.) (July 16, 1963), U.S. Pat. No. 3,491,504 (Young et al) (Jan. 27, 1970), U.S. Pat. No. 3,545,163 (Mahaffy et al) (Dec. 8, 1970), U.S. Pat. No. 3,694,991 (Perdue et al) (Oct. 3, 1972), and U.S. Pat. No. 4,537,011 (Bortolani et al) (Aug. 27, 1985). The disclosures of these patents are hereby incorporated by reference. However, it is noted that the film of the invention may be employed with various other types of skin packaging machinery, such as the VS-44 marketed by the Cryovac Division of W. R. Grace or the PORT-A-VAC marketed be AmPak. The film of the invention is not intended to be limited to skin packaging employing only the machinery mentioned herein.
Skin packaging techniques are often disadvantageous in that an excess portion of film is undesirably wrinkled on evacuation and the packaged material or article becomes irregular in shape, thus producing a poor external appearance. In addition, the wrapping film is apt to be broken in the portions folded and wrinkled. It has been ascertained in actual practice that as the film thins in the course of the vacuum skin packaging process, conventional films, due to poor tensile strength and poor elongation, tend to be broken when subjected to high stretch or draw ratios at particular projections or recesses randomly arranged on the material being packaged. The breaking can occur during the packaging or afterwards during handling of the packaged product. Methods and films are thus unfavorable from a practical point of view. There is accordingly a strong demand for improvement in the packaging films and methods.
Conventional low density polyethylene (LDPE) is typically used for skin packaging, and attempts to provide specific improved skin packaging films have been made in the past. For instance, disclosed in U.S. Pat. No. 4,275,544 (Hisazumi et al assignors to Kureha) (June 30, 1981) is an improved method of vacuum packaging by using a specific laminated film as a package film, the laminated film being composed of a plastically deformable layer of a synthetic resin having a yield stress more than 50 kg/cm.sup.2 at a temperature of 23.degree. C. and an elongation of less than 30% at elastic limit at a shaping temperature of 50.degree. to 180.degree. C. and an elastic layer of a synthetic resin having an elongation of more than 40% at elastic limit at the shaping temperature. Also, disclosed in U.S. Pat. No. 4,375,851 (Paulos assignor to Bemis) (Mar. 8, 1983) is a skin packaging material composed of a film of thermoplastic material having a reinforcement consisting of strands (in the form of netting, or all extending in one direction parallel to one another) adapted on exposure for an appropriate time interval to a source of heat to have both the film and strands heat up to a forming temperature for being skin packaged around an article on a substrate, with the strands less exstensible than the film on equal heating of the film and strands, and the skin packaging of articles utilizing said material in such manner that the strands flare out from the article at the base of the article to provide a broader base of support for the article and to prevent undue thinning of the film. Also, it is known from U.S. Pat. No. 4,590,741 (Long et al assignors to General Dynamics) (May 27, 1986) to skin package electrostatically sensitive items with a 3-layer film including first and second outer layers of heat-sealable, electrically semi-conductive polyethylene each having a surface resisivity exceeding 1.times.10.sup.16 ohms per square, and a middle layer of heat-sealable polyethylene sandwiched between and bonded to said first and second outer layers and impregnated with a sloughable, electrically-conductive material providing said middle layer with a volume resisivity no more than 1.times.10.sup.3 ohms per centimeter, wherein said electrically-conductive material is prevented from sloughing to contaminate said item by said first or said second semiconductive polyethylene layer.
Accordingly, an object of this invention is to provide a suitable film for skin packaging of products, which film exhibits high tensile strength and elongation properties even at high stretch or draw ratios, whereby it is sufficiently deformable so that it can conform to the shape of the high-profile and/or heavy product being packaged by deformation without breaking. In other words, the film possesses an excellent deep draw feature affording a high draw ratio. It is another object of this invention to provide a skin packaging film for packing high profile and/or heavy products which film has antistatic characteristics. Thus, it is an aspect of the film that the film serves well as a material for packaging articles which typically have sharp projections such as bone-in meat, electronic components, and the like. The films in accordance with the present invention also typically have excellent see-through properties which is advantageous for reading code numbers preprinted on a product wrapped with the film. The films often exhibit very good adhesion to the support, particularly thermal bonding when the support is surlyn-coated cardboard.